A semiconductor memory device is classified as a volatile memory device or a nonvolatile memory device.
The volatile memory device performs a high speed data read and write operation, but the stored data is lost when the power is cut off. The non-volatile memory device operates at relatively lower write and read speeds, but the stored data is preserved even when the power is cut off. Therefore, the non-volatile memory device is used to store data that is to be maintained regardless of power supply. Examples of the non-volatile memory device include a read only memory (ROM) device, a programmable ROM (PROM) device, an electrically programmable ROM (EPROM) device, an electrically erasable and programmable ROM (EEPROM) device, a flash memory device, a phase-change RAM (PRAM) device, a magnetic RAM (MRAM) device, a resistive RAM (RRAM) device, a ferroelectric RAM (FRAM) device, or the like. The Flash memory device is typically divided into a NOR device and a NAND device.
The flash memory device enjoys the advantages of both RAM and ROM devices. For example, the flash memory device can be freely programmed and erased similar to the RAM device. Similar to the ROM device, the flash memory device can retain the stored data even when they are not powered. The flash memory device has been widely used as the storage media of portable electronic devices such as mobile phones, digital cameras, personal digital assistants (PDAs), and MP3 players.
The size of a memory device is to be reduced in order to increase the degree of integration of a memory. However, semiconductor materials or processing conditions limit reduction in size of memory devices. Recently, there have been proposals of three-dimensional (3D) structured memory devices. In a 3D semiconductor memory device, cell current stability is a design consideration.